LED-based lamps have an ever-growing importance for lighting technology. The low power consumption and long lifetime of LEDs make them a highly favorable choice for various applications. White LEDs can be employed in retrofit bulbs that can replace conventional filament bulbs. Besides, LEDs having various colors are known in the art. It is also known to combine red, green and blue LEDs to create the impression of a virtually unlimited variety of colors.
Most retrofit LED bulbs sold today have on/off control, while others are dimmer compatible with phase cut wall dimmers. New developments are aiming at adding new functions to LED lamps, like wireless control by ZigBee or WiFi. For these applications an auxiliary supply is required to power the wireless communication chip, which increases costs as well as the complexity of the power supply design.
A colour-tunable LED bulb comprises three different LEDs (red, green and blue), wherein the light output of each LED is controlled individually to create different colours. A typical LED bulb with multiple PWM (Pulse Width Modulation) channels (such as R, G and B) requires a voltage-to-current converter and an auxiliary supply voltage for the control of the individual channels. High-end colour control requires an accurate control of the LED current. This is usually realized by an accurate current level and PWM. There are various ways to realize this LED PWM current.
For example, in a power conversion stage using a fly-back converter, i.e. with a typical transformer setup, mains AC may be converted to two DC voltages, 32V and 5V. The 32V voltage is then converted into a constant current with a buck converter; the setup is therefore also referred to as “dual-stage power conversion stage”. Since the constant current will be maintained even when the output voltage changes, it is possible to connect multiple LED channels in series and to control each channel by short-circuiting individual channels according to the required PWM (brightness) values.
Short-circuiting can be done by a so-called shunt topology, which is applied to control the light output. Herein, one or several LED channels connected in series may be present. Each of the LED channels comprises an LED, which is connected in parallel to a shunting switch operated to control the power consumption of the corresponding LED. In particular, the shunting switch may be a MOSFET.
For the fly-back converter of this exemplary setup, it is quite common to regulate the output voltage of the largest output power (32V of the LED circuit), derived from a main winding of the fly-back transformer. An auxiliary supply voltage (5V) is used to supply control electronics and may be derived from an auxiliary winding of the fly-back transformer. Due to the voltage equilibrium of the transformer windings and the shunt switching topology, variations of the auxiliary supply voltage may occur upon a change of the current in the main winding. Hence, if a shunt switching topology is employed, a constant voltage to supply control electronics, such as a microcontroller or the like, cannot be maintained by an auxiliary winding tapped off from the fly-back transformer.
Additionally, there is also another challenge: to have a floating supply voltage to drive the shunt switches.
Given the abovementioned problems, it is an object of the present invention to provide means for extracting an auxiliary power supply from a current source in order to control shunt switching with a cost-effective, loss-reducing setup.